1. Field of the Invention
The present invention relates to a method for manufacturing an optical fiber having a glass core rod and a cladding glass layer clothing the glass core rod manufactured either successively or continuously by using a carbon dioxide gas laser beam.
2. Description of the Prior Art
An optical waveguide having a doped fused silica glass core and a glass cladding layer formed of a pure fused silica or doped fused silica has been known. The method for manufacturing such an optical waveguide is known, for instance, by U.S. Pat. No. 2,326,059 in which a flame hydrolysis process is applied. In one known method for manufacturing such an optical fiber, a doped fused silica core rod which is doped in an amount to increase its refractive index by a desired level is inserted into a pure fused silica tube and the combination is heated up to a temperature to cause a sufficient elasticity to be achieved, then the combination of the tube and the rod is drawn to decrease the cross-section in a desired dimension at which the tube is collapsed about the rod and is fused together with the core. In another known method, a pure fused silica tube is used and solid particles of silicon oxide and that of the oxide of dopant are deposited on the inner wall of the tube and the tube is heated above its softening temperature and extended by drawing. In this method the cross-section of the tube is decreased while sintering the solid particles on the inner wall to form a solid optical fiber.
In the flame hydrolysis process for the manufacture of the glass core and the cladding layer, however, it is known that a water content of about 1,000 ppm may often be contained in glass which causes trouble in the practical use. Machining or surface working of the glass core rod may cause contamination by an impurity in the glass or it may result in variation of the composition at the surface layer of the glass which results in a scattering loss of the optical waveguide.